--------------------
-- Conversion of infix to postfix (W/O checking validity)

-- different core f's as we build complexity

post cs = f cs ""

-- simple predicates and other utilities

isAdd  = (`elem` "+-")
isMul  = (`elem` "*/")
notPar = (/='(')

(&) p q x = p x && q x

space = foldr (\x y->x:' ':y) ""


--------------------
-- Stupid version (no spaces, only addops)
{-

f (c:cs) s | isDigit c = c : ' ' :  f cs s
           | isAdd   c = space s ++ f cs [c]
f [] s = space s

-}


--------------------
-- Simple version (no parens, single digit)
{-

f (c:cs) s | isSpace c = f cs s
           | isDigit c = c : ' ' : f cs s
           | isMul   c = let (m,t) = span isMul s in
                         space m ++ f cs (c:t)
           | isAdd   c = space s ++ f cs [c]
f [] s = space s

-}


--------------------
-- Complex version (precedence and parens)
-- {-

f ('(':cs) s = f cs ('(':s)
f (')':cs) s = let (o,t) = span notPar s in
               space o ++ f cs (drop 1 t)
f (c:cs) s | isSpace c = f cs s
           | isDigit c = let (n,ds) = span isDigit cs in
                         c:n ++ ' ' : f ds s
           | isMul   c = let (m,t) = span (isMul & notPar) s in
                         space m ++ f cs (c:t)
           | isAdd   c = let (o,t) = span notPar s in
                         space o ++ f cs (c:t)
f [] s = space s

-- -}